Fuel oil which is used to heat residences and buildings is generally stored in small tanks located either inside the building, outside the building or in the ground near the building. In order to insure that an adequate supply of fuel is available to the building furnace, each residential tank must be periodically refilled by making a fuel oil delivery to the tank location Such fuel oil deliveries are presently made by a central distributor utilizing small tank trucks.
A problem arises in this rather simple supply system in that residential fuel oil tanks must be filled before the supply of fuel runs out yet it is uneconomical for the fuel oil distributor to refill the tanks on a set schedule especially during the warmer seasons in which fuel oil consumption is at a minimum. Accordingly, the present practice of fuel oil distributors is to schedule a delivery to a particular tank based on past usage history and recent weather conditions. This type of delivery is typically termed a "degree-day" system and uses mathematical algorithms to predict the amount of fuel-oil consumed by each fuel-oil user.
Unfortunately, in many residences the pattern of usage varies widely and the predictive algorithms which are used to determine when a delivery must be made are highly imprecise. Thus, it has been necessary to plan for a substantial reserve capacity for each tank, thereby reducing the amount of fuel-oil which can be delivered in each refill trip. In addition, recent trends in energy conservation practices by consumers and alternate energy sources such as solar energy have made fuel oil consumption predictions based on past history even less reliable than they have been in the past.
Consequently, fuel oil distributors have found the average amount of fuel oil delivered to the residential customers has been slowly decreasing. Since a substantial cost is incurred in personnel and equipment costs in making each delivery, the costs of operating a fuel-oil distribution center have risen substantially.
Accordingly, a number of prior art systems have been developed in order to monitor the fuel-oil level in residential tanks. Generally, these systems communicate fuel-oil level information from a remote sensing unit to a central receiving station generally by means of the direct dial telephone network. In the central receiving station the information is processed and a report is generated so that a delivery of fuel-oil can be scheduled to replenish the customer's supply before it runs out. Since the remote sensing unit is designed to report when the fuel-oil level on the customer's tank has fallen to a predetermined level, such systems insure that the fuel-oil deliveries made to the residential tanks will always be constant amounts and it is possible to reduce the tank reserve capacity and deliver larger quantities of fuel-oil on each delivery.
Two types of remote signaling systems are commonly in use. In a first type of prior art system, the central monitoring location initiates a telephone call to each remote location to gather fuel-oil information. However, when such systems are used in monitoring fuel-oil tanks located in residences, the incoming telephone call often proves annoying especially if it is at an inconvenient time. An example of such systems are shown in U.S. Pat. Nos. 3,899,639 and 4,147,893.
Accordingly, for residential use, other systems have been designed which locate a sensor/signaling unit at each residential location. This unit responds to an reorder condition created by either a sensed low fuel-oil condition at the residential location or by a timer and generates a telephone call from the residential location to the central unit after which information is transferred between the two locations. Some of these systems inconveniently immediately seize the telephone line even if a conversation is being carried on. Other systems may be arranged to test the telephone lines before use in order to avoid interrupting an ongoing call at the resident's location.
Examples of such systems are shown in U.S. Pat. Nos. 3,588,357; 3,842,208; 4,059,727 and 4,486,625. These systems can accurately monitor the residential fuel-oil level.
However, in a typical sensing system such as that described in the above patents, a single central monitoring station is in communication with a large number of sensor units. Typically, the central unit at a fuel-oil distributor's location may monitor hundreds of local fuel-oil tanks. Accordingly, it is desirable to make the remote sensing unit as inexpensive and as easy to install as possible in order to reduce the cost of tee overall system. Since there are only a few central location units however, the cost of these units can be more expensive.
Three of the chief requirements for reducing the cost of the remote sensing units are an inexpensive fuel-oil level sensor, an inexpensive mechanism to transmit information over the direct dial telephone number to the central location and a construction which allows the unit to be easily and quickly installed.
With regard to the first requirement, conventional systems often require dedicated level transducers in addition to those already present in the tank or flow meters which monitor the fuel which is used from the fuel-oil tank. Such units thus require the installer to make several connections to the fuel-oil system and increase installation time.
With regard to the second requirement, transmission over the direct dial telephone network involves a certain amount of interference and noise and, thus, any data transmission arrangement between the remote and central locations must be capable of reliably sending and receiving information in the presence of such noise.
Conventional transmission systems operate by modulating a precision carrier frequency (utilizing either frequency modulation, frequency-shift keying or pulse modulation). While such systems can operate reliably in the presence of significant amounts of noise and interference, they generally require a highly-stable oscillator at the remote location to produce the carrier signal. With present technology, such a precision, stable-frequency oscillator requires the use of expensive crystals and compensation circuitry to maintain the oscillation frequency constant even though the environment in which the oscillator is located may change substantially.
Finally, conventional systems are often time consuming to install because they require external power connections and level sensing devices must be added.
Accordingly, it is an object of the present invention to provide a fuel-oil reporting system which does not require a highly-stable oscillator.
It is another object of the present invention to provide a fuel-oil reporting system in which the remote sensing units are simple in construction and low in cost.
It is yet another object of the present invention to provide a fuel-oil reporting system which utilizes a special data transmission format that overcomes variations in the oscillator frequency in the sensing units.
It is a further object of the present invention to provide a fuel-oil reporting system which utilizes a simple and economical sensor to sense the fuel-oil level at the tank.
It is still a further object of the present invention to provide a fuel-oil reporting system which can be quickly and easily installed in new and existing tank locations.
It is yet a further object of the present invention to provide a fuel-oil reporting system which does not require an external power supply.
It is another object of the present invention to provide a fuel-oil reporting system which can report the occurrence of other alarm conditions such as low temperature or tank leaks utilizing the same transmission circuitry that is used to report fuel-oil level.